Where does the soil end and the atmosphere begin?

Many think that there is a clear line between soil and air but that is so very from the reality.

The soil breathes and the tidal forces that pull water up through the soil and then dives back in act as the soil's diaphragm, just on a much longer time scale. The ruminant eats grass low to the ground and ingests soil bacteria which find a fruitful environment for reproducing itself on an exponential scale in what is simply a self-propelled bioreactor. Then the ruminant deposits a fresh batch of digested cellulose and soil bacteria on the ground. That, in turn, is taken deep into the soil by dung beetles and other critters.

Where does the soil end and the atmosphere begin? I'll give it a number: the air begins four feet below the soil surface and the soil ends 4 feet above.

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